The conversion of fuel to energy defines technology at the center of one of the most important industries in existence. Most energy conversion in this arena involves the combustion of fuel to produce mechanical, thermal, and/or electrical energy. Coal, oil, and gasoline are fuels typically used in conventional combustion technology. The combustion of these fuels (burning) involves applying enough heat to the fuel, in the presence of an oxidant such as the oxygen in air, for the fuel to undergo a relatively spontaneous and ill-defined combustive, often explosive, reaction in which chemical bonds in the fuel break and reactions with oxygen occur to produce new compounds that are released into the environment (exhaust). In the process, energy is released in the form of heat and an expansive force, which can be used to drive a piston, turbine, or other mechanical device. This mechanical energy can be used directly, e.g., to drive an automobile or propel a jet aircraft. It also can be converted into electrical energy by linking the mechanical device to an electrical generator. Or it can simply be used to provide heat, e.g., in a home.
Fuel combustion is, as noted, relatively ill-defined. That is, the precise chemistry occurring during combustion is not well known or easily controlled. What is known is that the resulting exhaust typically includes a wide variety of toxic compounds such sulfur-containing toxins, nitrous compounds, and unburned fuel droplets or particles (soot), some of which can be converted by sunlight into other toxins such as ozone, as well as a significant amount of carbon dioxide which, while not toxic, is an important greenhouse gas that many experts believe is affecting the environment.
Cutting edge research and development in the area of energy conversion generally is aimed at improving efficiency and/or reducing the emission of toxic pollutants and greenhouse gases. Fuel cells represent a significant advance in this area. Fuel cells are generally very clean and efficient, and also are very quiet, unlike most combustion engines and turbines. Fuel cells convert fuel directly into electrical energy via a relatively well-defined, controllable, electrochemical reaction that does not involve explosive combustion. In some systems, the only reaction product exhausted into the environment is water. In electrical production, no intermediate mechanical device, such as a piston engine or turbine, is needed, thus the process is generally much more efficient, since intermediate mechanical devices cause significant energy loss through friction, etc. The efficiency of conversion of fuel to mechanical energy via combustion in a piston engine is also hampered by the laws of physics; the Carnot Cycle, via which piston engines operate, determine the limit of efficiency in the conversion of heat, from combustion, into mechanical work. Significant loss of energy is unavoidable.
While fuel cell technology has been developed to some extent, it has not assumed a significant role in worldwide energy conversion. Significant improvements are likely needed for this to happen.